1. Field of the Invention
The present invention relates to a metallic printed board and in particular to a metallic printing board having a multiple layer structure including a metallic plate, an electric insulating material layer and a metallic film.
2. Description of the Prior Art
As for prior art metallic printed boards (called metallic boards), electric insulating materials developed on an identical surface on the metallic board are composed of a single kind of material. Regions on which electronic circuits are defined on the metallic board are formed either only on a single side surface of the metallic board or on both side surfaces of the metallic board, but both of these single-sided regions and double-sided regions are not simultaneously placed on the metallic board. In addition, the metallic board is never used as a case of the apparatus using the metallic board or a structural member of the apparatus. FIGS. 1, 2, 3 and 4 illustrate the exemplary structure of the above described prior art metallic board.
FIG. 1 shows a layout where two individual circuits having different specifications are developed on individually separated metallic boards and the separated metallic boards are connected to each other by cables. With respect to insulating materials, the metallic board 1 may be a member with low dielectric constant and the metallic board 2 may be a member with low thermal resistance. Connectors 9 are mounted on the metallic boards 1 and 2, between which cables 10 are connected.
FIG. 2 shows a cross-sectional view of an apparatus in which two kinds of metallic boards with different properties are installed. In FIG. 2, the low thermal resistance metallic board 2 and a complementary board 11 are placed separately and connected by the spacer-f 20, and screws 21 fix the boards 2 and 11 at the spacers 20. In order to obtain heat radiation efficiency, a metal plate of the metallic board 2 is placed in contact with the metallic case 12.
FIG. 3 shows a cross-sectional view of the metallic board where the metallic board 2 is formed as a single-sided packaging board to be contacted to the cooling fins 14. FIG. 4 is a cross-sectional view of the metallic board where the metallic board 15 is formed as a double-sided packaging board and he be fixed to the metallic case 12 by screws 22.
In the above described prior art metallic boards, insulating materials thereof are composed of a single kind of material. Since the insulating material used for the metallic board is selected by considering its specific purposes and specifications with respect to used voltages, frequencies, and power losses, the metallic board is formed individually to satisfy specific requirements. For this reason, the following problems occur.
(1) In forming circuits with different specifications, various kinds of metallic boards separated in relatively small sizes are required in a single frame of an apparatus.
(2) Heat transmission for releasing generated heat cannot be fully established in separated metallic boards, and therefore, cooling fans are necessary for metallic boards which generate relatively large amounts of heat due to power losses, which leads to enlarging the size of the apparatus.
(3) A single-sided metallic board has a relatively high heat radiation efficiency, but the device packaging density is low. On the other hand, as for a double-sided metallic board, the device packaging density is relatively high, but the heat radiation efficiency is not relatively high, and therefore heat Iq removal devices are necessary at both ends of the metallic board which may lead to scaling up of the metallic board itself. Anyway, reducing the size of either metallic board is restricted.
(4) The size of the case for housing the various kind of metallic boards simultaneously increases.
(5) The increased number of connection parts such as connectors and cables due to the increased number of separated metallic boards may give rise to a relative reduction of the overall reliability of the apparatus.
In addition to the above described packaging problems where the insulating materials for the metallic board can only be of a single kind, mechanical problems generic to the metallic board itself may occur for the similar reasons. In using a single kind of material for the insulating materials of the metallic board, the heat radiation efficiency and the heat-proof capability cannot be optimized at the same time, that is they must be traded off. More specifically, in considering the optimization of the heat radiation efficiency, it is required to increase the content of filling materials used for the insulating materials in comparison with resin, which is the major content of the insulating materials. In this case, as the content of resin is reduced, structural defects such as pin holes may tend to occur, and the voltage margin they can withstand may be reduced. In case of increasing the content of resin, the voltage margin may be increased and the heat radiation efficiency may be reduced leading to generation of large amount of thermal strain during a heat hardening process of the resin to bend the board.